Class D amplifiers generate output voltages that may vary different output voltage levels. In particular, Class D amplifiers generate pulse modulated output voltages to drive load devices in response to amplitude variations of an input signal. Conventional Class D amplifiers are often implemented using a transistor H-bridge to generate switched output voltage levels.
FIG. 1 illustrates a conventional transistor H-bridge 100 that may be used in a Class D amplifier. A conventional transistor H-bridge 100 includes a pair of PMOS transistors MP1 and MP2 and a pair of NMOS transistors MN1 and MN2. The PMOS transistors MP1 and MP2 each have sources coupled to a first supply potential, illustrated here as VDD. The NMOS transistors MN1 and MN2 each have sources coupled to a second supply potential, illustrated here as GND. Transistors MP1 and MN1 are drain coupled at a first output node OUT1, and transistors MP2 and MN2 are drain coupled at a second output node OUT2. The transistor H-bridge may be coupled to a load 110 at the respective drain connections for MP1/MN1 and MP2/MN2. An H-bridge controller (not shown) may be coupled to the gates of each transistor and control switching of the transistors to output three voltage levels across the load 110. In certain audio applications, the load 110 may be an audio speaker. As a signal input to the class D amplifier changes in amplitude, the controller turns off and on the switches of the H-bridge to generate corresponding switched output voltage levels across the load. The three output voltage levels include +VDD, −VDD, and 0V.
During operation, transistor MP2 is switched on while transistor MN1 is held on to drive the +VDD voltage level across the load 110. To drive the −VDD voltage level across the load 110, transistor MP1 is switched on while transistor MN2 is held on. To drive 0V across the load 100, either MP1 and MP2 are switched on or MN1 and MN2 are switched on.
Class D amplifiers using conventional transistor H-bridges generally operate at approximately 90% efficiency. This efficiency level may be too low for portable applications such as consumer electronics devices because it negatively impacts battery life of the portable devices.
Voltage boosters may be used in Class D amplifiers to increase amplifier output power. A voltage booster boosts the VDD supply voltage coupled to transistors MP1 and MP2 in order to drive a voltage higher than the supply voltage VDD across the load 110. However, such boosters also operate at approximately 90% efficiency. The overall efficiency of a boosted Class D amplifier may therefore be decreased to an efficiency of approximately 81%, thus further decreasing battery life.
Accordingly, there is a need in the art for a voltage boosted Class D amplifier having increased operating efficiency.